Field of the Invention
The invention relates to the forming of containers from parisons made of plastic material by blow molding or stretch blow molding in a mold bearing the imprint of a model of the container to be formed. The term “parison” refers to not only a preform (ordinarily obtained by injection), but also an intermediate container that has undergone a preliminary blow molding operation starting from a preform.
Description of the Related Art
A mold usually comprises multiple one-piece elements, each having a molding surface bearing the imprint of at least a portion of the container. Thus, a mold that is designed for forming containers comprises a side wall bearing the imprint of a body and a shoulder of the container, generally divided into two mutually articulated half-molds for making it possible to insert a parison into the mold and a mold bottom bearing the imprint of a bottom of the container, with this mold bottom being positioned in an opening made between the half-molds opposite the shoulder. For some containers having particular local shapes (for example protrusions or recessed reserved places forming a handle), the mold can comprise movable inserts bearing the imprints of these shapes.
The molds are generally regulated thermally in such a way as to keep them at a stable temperature making it possible either to cool the containers at the end of forming (general case), or, in certain cases, in contrast to heat them (case of the containers designed to be filled with hot contents—this is then called thermofixation, or heat-setting in English).
In the two cases, a well-known technique consists in making a fluid circulate in the mold (in gaseous or liquid form, in general water or oil) via a fluid circuit that is partially formed in the mold elements (half-molds, mold bottom, possible inserts, supports). For cooling, the fluid is introduced into the fluid circuit at a relatively low temperature, typically on the order of 10° C. For heating, the fluid is introduced into the fluid circuit at a relatively high temperature, typically on the order of 140° C. In the case of heating, an alternative technique for thermal regulation consists in integrating electrical resistors into the wall of the mold, as proposed in the international application WO 2013/093335 (Sidel Participations).
It is easily understood that the thermal regulation is more effective the larger the exchange surfaces (defined by the fluid circuit) between the coolant and the material of the container. A common objective, for the manufacturers of molds, is therefore to maximize the exchange surfaces defined by the fluid circuit. More specifically, for reasons of rationalization of the manufacturing of molds and limitation of the loss of material, the objective is to maximize the surface/volume ratio of the fluid circuit. This objective, however, comes up against multiple limitations, in particular:                Machining constraints (linked to the techniques used: molding, turning, milling, electroerosion, etc.),        Manufacturing tolerances associated with the necessary sealing of the fluid circuit with the interfaces between the different parts composing the mold elements,        The mechanical strength of the molds (which is advantageously maximized),        The consumption of coolant (which is advantageously minimized),        The space requirement constraints, with the environment of a blow molding mold being particularly bulky.        
The mold bottom is a good nexus of the problems arising from heat regulation because the shape of its imprint is generally complex, its volume is limited, and it is often independent of the half-molds (which involves an independent fluid circuit with its own feed and drain pipes connected to hoses). A typical example of the mold bottom is presented in the document EP 0 742 094 (Asahi): this mold bottom is equipped with a fluid circuit (in this case for cooling), of which it is indicated that it is machined as close as possible to the surface in such a way as to maintain good cooling efficiency.
The technique that consists in penetrating the transverse channels in the bottom (which it is then necessary to connect) may be suitable when, as in the example illustrated in this document, the impression surface is flat. By contrast, as soon as this surface is raised, it is no longer possible to bring the channels close to the surface, at the risk of running into the latter. At best, it is possible, by penetrating the channels both obliquely and radially, to follow the raised pattern roughly (and only locally), as illustrated in the U.S. patents U.S. Pat. No. 5,971,740 (Rees) and U.S. Pat. No. 5,762,981 (Wentworth). This technique, however, does not make it possible to carry out homogeneous heat exchanges, with the parts of the bottom of the container located perpendicular to the channels enjoying better cooling (or conversely better heating) than the parts that are offset angularly from the channels.
Another technique, designed to improve the efficiency (and more specifically the homogeneity) of the heat exchanges, consists in making in the mold bottom, by milling, a single groove that has concentric circular portions connected by radial straight portions to form, roughly, a single channel in a continuous coil shape in which the fluid enters through a central opening to exit from it via a peripheral opening.
Such a structure, presented in the document U.S. Pat. No. 7,025,584 (Wentworth), which also calls for connecting a dividing plate equipped with projecting pegs designed to form baffles in the channel, is not without its drawbacks either, however.
In the first place, in the case of a molding surface lacking symmetry of revolution, the coil shape of the channel does not solve the problem of the lack of homogeneity of the heat exchange. Typically, in a mold bottom bearing the imprint of a petal-shaped bottom, whose raised pattern is particularly complex, the raised patterns that correspond to the feet, closer to the channel, inevitably enjoy a better heat exchange than the raised patterns corresponding to the valleys, relatively farther from the channel.
In the second place, the production in a coil of the channel (instead of a series of radial channels) induces a gradual reduction of the heat capacity of the fluid along the channel, where the zones whose distance (measured in the curvilinear abscissa along the channel) from the central opening is small offer better heat exchange capacity than the zones whose distance from the central opening is relatively larger. This geometry does not pose a problem in the central zone of the bottom, which should in general enjoy maximum heat exchange, regardless of whether it is to cool it, as in the case of an ordinary container designed for plain water or, by contrast, to heat it, as in the case of a heat-set container. By contrast, this geometry poses problems in the area of similar peripheral zones (for example, the impressions of feet in a petal-shaped bottom), whose radial distance from the center of the bottom is identical and which should receive from (or give to) the coolant the same number of calories, but which because of their different curvilinear distance from the center (measured along the channel), receive (or give) a different number of calories taking into account the gradual exhaustion of the heat capacities of the coolant.
In the third place, the sealing of such a mold bottom is difficult to ensure. Admittedly, seals (in particular O-ring seals) are inserted into the interfaces between the different parts, but taking into account heat cycles, these seals undergo an accelerated fatigue that results in leaks. This gives rise to frequent maintenance operations that interrupt the production.
In the fourth place, the manufacturing of this mold bottom is long and complex. At a minimum, it involves a first operation for producing a parison (by molding or by machining) of the one-piece mold bottom element, a second operation for producing (by milling) the groove, a third operation for producing (by machining) a connected plate serving as a cover to the fluid circuit, and then a fourth operation for assembly (by screwing) of the plate on the one-piece element with at least one seal being inserted.
Several objectives are consequently targeted, individually or in a group:                Improving the homogeneity of heat exchanges in a mold bottom and even, preferably, ensuring these heat exchanges according to a predetermined profile regardless of the raised pattern of the molding surface,        Maximizing the yield of the heat exchanges for purposes of achieving energy savings, improving the quality of the containers produced, and reducing the cycle time;        Improving the sealing of the mold bottom;        Simplifying and accelerating the manufacturing of the mold bottom.        